High frequency coupling system



Oct. 28, 1958 A. B. I EvlN HIGH FREQUENCY COUPLING SYSTEM Filed Dec. 2, 1954 INVENTOR. v HH 5W/7 United States Patent C) i HIGH FREQUENCY coUPLING SYSTEM Aaron B. Levin, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Penn- Sylvania The invention relates generally to signal transmission means and more particularly to high frequency signal duplexers.

The copending application of William E. Bradley, Serial No. 271,477, filed February 14, 1952, now Patent No. 2,795,761, discloses a form of single tube heterodyne converter which employs a two cavity klystron. The electron stream of the klystron is modulated at two different frequencies before entering the drift space between the input cavity resonator and the output cavity resonator. The electron stream is modulated at a low frequency at the cathode or grid of the klystron and again at a higher frequency in the input cavity resonator of the klystron'. The converter disclosed in the copending application mentioned above differs from the usual form of heterodyne converter in that the signal supplied to the input cavity resonator is obtained from the output cavity resonator rather than from a separate oscillator circuit. The useful output signal from the converter is a signal at a frequency equal to the sum or differenqce of the high frequency and the low frequency. This sum or difference frequency signal is also obtained from the output resonator of the klystron. The fact that two signals are extracted from the single output cavity resonator presents certain problems n'ot normally encountered in heterodyne converter circuits. The ow of electrons through the resonator gap excites a signal in the resonator only if the gap presents a high impedance to the flow of electrons thereacross at the frequency of that signal. Normally the output resonator is tunable to only one frequency at a time. It is only at this frequency that a high impedance is presented across the resonator gap. Therefore, in the usual form of heterodyne circuit, it is only at this one frequency to which the cavity resonator is tuned that energy may be extracted from the electron stream. The copending application discloses one arrangement of output coupling elements which permits two signals to be extracted from the modulated electron stream.

It is an object of the present inven'tion to provide improved means for obtaining two separate signals from a doubly modulated electron stream.

More particularly, it is an object of the present invention' to provide an improved arrangement of circuit elements which permits signals at two frequencies to be extracted from the output cavity resonator of a velocity modulated type tube.

It is a further object of the present invention' to provide an output coupling circuit which separates into two different channels the two signals extracted from a modulated electron stream.

Another object of the present invention is to provide sign'al duplexing means which, when used in conjunction' with an appropriately tuned cavity resonator supplied with an appropriately modulatde signal, will extract two signals therefrom and channel them to separate outputs of the signal duplexer.

Patented Oct. 28, 1958 ice These and other objects of the invention are generally accomplished by including resonant circuit elements in the signal duplexer which selectively block or pass the signals of the two different frequencies. The location of these resonant elements are so chosen with respect tothe cavity resonator that a resonant condition is induced therein at least one frequency other than the natural resonant frequency thereof. When an appropriately modulated signal is supplied to the resonator, the signals at two different frequencies are obtained from separate outputs of the duplexer.

For a better understanding of the invention together with other and further objects thereof reference should now be made to the following detailed description which is to be read in conjunction with the accompanying drawings in which:

Fig. 1 is a schematic drawing of a preferred embodiment of the invention;

Fig. 2 is an equivalent circuit of the embodiment of Fig. 1 for one of the two -frequencies of operation' of the system; and

Fig. 3 is an equivalent circuit of the embodiment of Fig. l for the other of the two frequencies of operation.

In Fig. 1 a cavity resonator 10 is provided for extracting energy from a doubly modulated electron stream passing through electron permeable grids 12 and 14. Cavity resonator 10 is shown as having a toroidal shape, this being the shape of the output cavity resonatorof the S. A. C. 19 klystron tube which has lbeen used successfully as a single tube frequency converter. In the operation of the invention, a stream of electrons so generated as to have a modulation impressed thereon at two separate high frequencies is passed through grids 12 and 14. The source of this stream of electrons may be a traveling wave tube or any one of a number of electron stream modulating devices. However, in the present description it will be assumed that it originates in a klystron modulator tube 15 in which resonator 10 forms` the output cavity reson'ator. The modulator tube also includes an input cavity resonator 16, cathode 18 and a collec tor 19. Source 17 supplies a low frequency signal which may be modulated in frequency, amplitude or phase. Source 17 is connected in a manner tovary the fixed potential existing between cathode 18 and input resonator 16. As explained in detail in the above-identified co' pending application, the output signal obtained from resonator 10 will have impressed thereon the modulation of the low frequency sign'al.

An output waveguide 20 is coupled to cavity resonator 10 through an iris 22 formed in the outer wall of the resonator. Waveguide 2t) may be a rectangular waveguide of appropriate dimension for the frequencies gen erated in the heterodyne modulator circuit. The vertical direction in Fig. l corresponds to the narrow dimension of the waveguide 20. A second waveguide 24 is coupled to waveguide 20 in an E-plane or series junction. Wave guide 24 is coupled to an output waveguide 26 through a waveguide baudpass filter 23. The length of waveguide 24 from the series junction to the input of the bandpass lter 28 is designated as L3 in' Fig. 1. Filter 28 is so constructed that it matches the characteristic impedance of line 24 at the frequency of the signal to be transmit ted to line 26. For convenience this signal will be referred to hereinafter as the output signal, and the other signal to be extracted from resonator l@ will be referred to as the local oscillator signal. Filter 24 appears as a large shunt susceptance or a good short circuit at the frequency of the local oscillator signal. A bandpass filter of this type may comprise a plurality of rectangular cavities in cascade or it may comprise a waveguide with a plurality of cavity resonators coupled thereto at appropiate intervals. A filter of the first type is described in the Journal of Applied Physics, volume v18, October 1947, page 862. A filter of the latter type is shown in U. S. Patent No. 2,484,798 to W. E. Bradley, assigned to the assignee of the present invention.

An inductive iris 3) is provided in waveguide 20 at a distance L1 from iris 22. This iris 22 acts as an inductive susceptance which is chosen so as not to match the characteristic impedance of waveguide 20. Length 'L1 `is chosen to be an integral number of half wavelengths o'f the local oscillator signal. A tuning screw 31 is provided in waveguide 20 at a point between Vthe series junction and resonator 10.

A cavity resonator 32 terminates waveguide 20 at a distance L2 beyond iris 30. Resonator 32 is perferably a high Q cavity resonator which matches the characteristic impedance of waveguide 20 at the frequency of the local oscillator signal. A second tuning screw 33 isprovided in waveguide 2G approximately midway between iris 30 and resonator 32.

The structure thus far described, and excluding resonator 16, cathode 1S and collector 19, forms 4the basic structure of the present invention. Two signals may be extracted from a modulated electron stream passing through grids 12 and 14 and these two signals willappear entirely separated each from the other 'at waveguide 26 and at the output of resonator 32, respectively. However, to complete the single tube modulator circuit chosen as an example of a preferred application of the present invention, it is necessary to provide a waveguide section 34 which couples cavity resonator 32 to the input cavity resonator 16 of the velocity modulated tube. An adjustable line section schematically represented by the arrow 36 is provided in waveguide 34 for adjusting the phase of the signal supplied to input cavity resonator 16. A

second output coupling means may be provided in resonator 32 or waveguide 34 for supplying an external circuit with a signal of the same frequency 'as the one supplied to resonator 16.

In the operation of the system of Fig. l as a single tube heterodyne converter, input resonator 16 is tuned to the frequency of the local oscillator signal. The passage of the electron stream through resonator 16 causes the stream to be modulated at the frequency of the local oscillator signal. As indicated in the above-mentioned copending application, the electron stream may be modulated at the low frequency by varying the potential of cathode 18 with respect to input cavity resonator 16. The twice modulated electron stream reaches resonator through the usual drift space. Upon arrival at resonator 10 it is so bunched that it is capable of exciting in said resonator modulation components at frequencies equal to the local oscillator frequency plus or minus integral multiples of the low frequency. The modulation components at the local oscillator frequency plus or minus the first multiple of the low frequency will have the largest amplitude, but signals at the other frequencies representing higher multiples of the low frequency may 'be extracted from the electron stream if necessary or desirable. Resonator 10 is tuned to one of these frequencies, for example the local oscillator frequency plus the low frequency. The tuning of the resonator 10 determines the frequency of the output signal since it is only at the frequency to which resonator 10 is tuned that a high resistive impedance is present acrossthe gap between grids 12 and 14. As mentioned previously, the electron stream excites a signal in the resonator only if thev gap presents a high impedance in the flow of electrons thereacross at the frequency of that signal. It goes without saying that resonator 10 cannot be tuned to any arbitrary frequency but must be tuned to a frequency of one ofthe modulation components; otherwise there will be no component present in the modulated electron beam to excite the resonator 10 at its resonant frequency.

Having provided a resonator 10 tuned to ithe proper frequency to extract the output signal from the doubly modulated electron stream, it is then necessary to transmit this signal to output waveguide 26 with as little loss as possible. In the system of Fig. l this is accomplished by causing the impedance looking into the portion of waveguide 20 from to the right of the series junction to appear `as a short circuit. Resonator 32 is normally tuned to the local oscillator frequency so it will appear as a high susceptance (substantially a short circuit) at the frequency of the output signal. The distance L2 is chosen so thatiris 30 and -resonator 32 form a resonant circuit at the frequency of the output signal with a resulting high impedance at the iris 30. Tuning screw 33 may be adjusted to make this combination resonant at exactly the frequency of the output signal. Iris 30 is positioned approximately an odd number of quarter wavelengths from the series junction at the frequency of the output signal. The high impedance at the iris is reflected as a very low impedance at the junction. This low impedance, essentially a short circuit, is in series with the input impedance of waveguide 24. Since filter 28 is matched to waveguide 24 `at the frequency of the output signal, the input impedance of waveguide 24 will be its characteristic irnpedance. Under these conditions practically all of the energy at the frequency of the output signal will be channeled from waveguide 20 to waveguide 24. The output signal will pass through filter 28 to waveguide 26 with very little attenuation since filter 28 is tuned to pass a signal at the frequency of the output signal.

The equivalent circuit of the system of Fig. l at the frequency of the output signal is shown in Fig. 2. It is believed that this figure is self-explanatory when viewed in the light of the explanation of Fig. l given above.

Extracting a signal at the frequency of the local oscillator signal presents a different problem. Resonator 10 is not tuned to this frequency and, for this reason, modulation components at the local oscillator frequency ordinarily will not excite resonator 10. As indicated above, the reason why resonator 10 is not excited at frequencies off resonance is that the gap impedance is very low. However, this diliculty is overcome, in accordance with the present invention, by choosing the distance L1 to vbe an even number of half wavelengths at the local oscillator frequency. Iris 30 is so dimensioned that it provides a large shunt susceptance at the frequency of the local oscillator signal. The large shunt susceptance of iris 30 is refiected as a similar large susceptance at iris 22. If this susceptance is equal to the susceptance of resonator 10 but opposite insign, a second resonant condition is set up in resonator 10. Under these conditions the impedance across the gap between grids 12 and 14 will be high at the frequency of the local oscillator signal and energy will be extracted from the electron stream at this second frequency.

If the frequency of the local oscillator signal lies on one side of the natural resonant frequency of resonator 10 the input susceptance of resonator 10 will be capacitive, and if it lies on the opposite side of the natural resonant frequency the input susceptance will be inductive. However, for frequencies well away from the natural resonant frequency this input susceptance of resonator 10 is very large, essentially a short circuit, so the length L1 is approximately the same for the two vpossible conditions. The effective length of the transmission line can be changed by the small amount requiredby adjusting tuning screw 31. The energy extracted from the flow of electrons at the frequency of the local oscillator signal passes down waveguide 20, through cavity resonator 32. In the embodiment of the invention shown in Fig. l energy in waveguide 34 is returned to cavity resonator 16 in proper phase to sustain the modulation component at the local oscillator frequency at resonator 10.

yEnergy at the local oscillator frequency is excluded from waveguide 26 by the bandpass 'filter 28. Filter 28 appears as a short circuit to energy atfthe frequency of the local oscillator signal. The distance L3 is chosen to be equal to an integral number of half wavelengths at the local oscillator frequency. Thus the short circuit presented by filter 2S is reected as a short circuit at the series junction of waveguide 24 and waveguide 20. To minimize further the energytransmitted to waveguide 24, the distance from iris 30 to the series junction is made equal to an odd number of quarter wavelengths at the local oscillator frequency. Therefore the low impedance presented by iris 30 at the frequency of the local oscillator signal is reflected as a high impedance at the series junction. Reference to Fig. 3 will show that the low impedance presented by waveguide 24 is in series with the high impedance presented by the por, tion of waveguide 20 to the right of the junction. Therefore, the well known signal divider action causes substantially all of the energy at the local oscillator frequency to be channeled down waveguide 20 with practically no energy at this frequency passing into waveguide 24. In one model of the invention it was found that the local oscillator signal was more than 55 db below the level of the output signal in wave guide 26. This represents an improvement of over 35 db over the original circuit shown in the above-identied copending application.

It should be pointed out that it would lie within the scope lof the present invention to tune resonator to the local oscillator frequency and then refect the necessary impedance at the frequency of the output signal. Again it should be emphasized that reference to the two signals as local oscillator signal and output signal, respectively, should not be construed as imposing any limitation whatsoever on the invention. If resonator 10 is not apart of a single tube heterodyne converter but receives the modulated stream of electrons from some arbitary source or receives energy in some other manner, these two designations are equivalent to first and second signals, respectively.

The system shown in Fig. l represents what is at present considered to be the preferred embodiment of the invention. It Will be obvious to those skilled in the art that changes and modifications may be made therein Within the scope of the invention. For example, coaxial transmission lines may be substituted for the waveguides and different or additional tuning means may be inserted in the wave guides or coaxial lines. In addition, loop couplings may be substituted for the iris couplings shown in the drawings and other forms of reactive elements may replace iris 30. Therefore, reference should now be made to the hereinafter appended claims for an indication of the true scope of the invention.

Having now described my invention I claim:

l. A signal duplexer comprising a cavity resonator of selected bandwidth, said resonator being tuned to a first frequency, a first transmission line coupled to said cavity resonator, reactive circuit means coupled across said first transmission line, said reactive circuit means being positioned at a point remote from said resonator and so as to reflect a susceptance which, at the point of coupling of said transmission line to said resonator, is approximately equal in amplitude and opposite in sign to the susceptance of said resonator at a second frequency, said second frequency differing from said rst frequency by an amount substantially greater than one-half the said bandwidth of said resonator, a second transmission line coupled to said first transmission line in a series junction at a point intermediate said resonator and said reactive wavelengths at said second frequency whereby the im- 2. A signal duplexer for extracting energy at two frequencies from a modulated electron stream, comprising a cavity resonator of selected bandwidth having a pair of electron permeable grids through which said modulated stream of electrons may pass, said resonator being tuned to a first frequency, a first transmission line coupled to said resonator, reactive circuit means coupled across said first transmission line, said reactive circuit means reflecting a susceptance which, at the point of coupling of said first transmission line to said resonator, is approximately Vequal in amplitude and opposite in sign to the susceptance of said resonator at a second frequency, said second frequency differing from said first frequency by an amount substantially greater than one-half the said bandwidth of said resonator, said second transmission line forming a series junction with said first transmission line at a point intermediate said resonator and said reactive circuit means, filter means coupled to said second transmission line and functioning to pass signals at said rst frequency while blocking signals at said second frequency, said filter means being positioned approximately an integral number of half wavelengths at said second frequency from said junction of said first and second transmission lines thereby to cause the impedance of said second transmission line at said junction to be substantially a short circuit at said second frequency.

3. A signal duplexer comprising a cavity resonator of selected bandwidth, said resonator being tuned Vto a first frequency, a first transmission line coupled to said cavity resonator, means having a high susceptance at a second frequency, said second frequency differing from said first frequency by an amount substantially greater than one-half the said bandwidth of said resonator, said means being coupled to said first transmission line at a point approximately an integral number of half wavelengths from said resonator at said second frequency, a second transmission line joined to said first transmission line in a series junction at a point intermediate said resonator and said means, a band-pass filter coupled to said second transmission line and functioning to pass signals at said first frequency while presenting substantially a short circuit to signals at said second frequency, said filter being positioned an integral number of half wavelengths from said series junction at said second frequency.

4. A signal duplexer comprising a cavity resonator of selected bandwidth, said resonator being tuned to a first frequency, a first waveguide coupled to said cavity resonator, an iris having a high susceptance at a second frequency, said second frequency differing from said .first frequency by an amount substantially greater than one-half the said bandwidth of said resonator, said iris being positioned in said first waveguide at a point approximately an integral number of half wavelengths from said resonator at said second frequency, a second waveguide joined to said first waveguide in a series junction at a point intermediate said resonator and sald iris, a band-pass filter coupled to said second wavegmde and functioning to pass signals at said first frequency while presenting substantially a short circuit to signals at said second frequency, said filter being positloned an integral number of half wavelengths from said series junction at said second frequency.

5. A signal duplexer comprising a cavity resonator of selected bandwidth, said resonator being tuned to a first frequency, a first waveguide coupled to said cavity resonator, a first band-pass element coupled to said first waveguide, said band-pass element being tuned to a second frequency and presenting substantially a short circuit across said first waveguide at said first frequency, an iris having a high susceptance at said second frequency, said second frequency differing from said first frequency by an amount substantially greater than onehalf the said bandwidth of said resonator, said iris being positioned in said waveguide at a point intermediate said band-pass element in said resonator and approximately an integral number of half wavelengths from said resonator at said second frequency, the combination of said iris and said band-pass element being resonant at said first frequency, a second waveguide joined to said first waveguide in a series junction at a point intermediate said resonator and Vsaid iris and approximately an odd number of quarter wavelengths from said iris at said second frequency, a band-pass filter coupled to said second transmission line and functioning to pass signals at said first frequency while presenting substantially a short circuit to signals at said second frequency, said filter lbeing positioned an integral number of half wavelengths from said series junction at said second frequency.

6. A signal duplexer for extracting energy at two frequencies from a modulated electron stream, comprising a first cavity resonator of selected bandwidth and having a pair of electron permeable grids through which said stream of electrons may pass, said resonator being tuned to a first frequency, a second cavity resonator tuned to a second frequency, said second frequency differing from said first frequency by an amount substantially greater than one-half the said bandwidth of said first 'cavity resonator, a first waveguide lcoupling said firstresonator to said second resonator, an iris having a high susceptance at said second frequency, said iris being positioned within said first waveguide at a point approximately an integral number of half wavelengths from said first resonator at said second frequency, said iris being approximately an integral number of half wavelengths from said second resonator at said first frequency, said iris and said second resonator forming a combination resonant at said first frequency, a second waveguide joined to said first waveguide in a series juction at a point intermediate said iris and said first resonator and at a distance approximately an odd number of quarter wavelengths from said iris at said second frequency, a band-pass filter coupled to said second waveguide at a point approximately an integral number of half wavelengths from said series junction at es said second lfrequency, said lter functioning to pass signals atsaidfrst frequency while presenting substantially a short circuit to signals at said second frequency.

7. In a heterodyne modulator circuit including a velocity modulated electron .tube having an output cavity resonator which has a selected bandwith and is tuned 4to a flrst'frequency and a buncher cavity resonator tuned to a second frequency, said second frequency differing from said kfirst frequency by an amount substantially greater than one-half the said bandwidth of said -output cavity resonator, ran output coupling means comprising a third cavity resonatortuned to said second frequency, a first waveguide coupling said third resonator to said output resonator,an irisV presenting a high inductive susceptance at saidsecond frequency, Isaid iris being positioned in'said waveguide at a point intermediate said output resonator and said third'resonator and at a distance approximately an integral number of half wavelengths from Vsaid output resonator at said second frequency, the combination of said iris and said third resonator being resonant at said rst'frequency, a second waveguide forming a series junction with said first waveguide, said series junction being at a point intermediate said iris and said output resonator and at a distance of approximately an odd number of quarter wavelengths from said iris at said second frequency, ya band-pass filter coupled to said second waveguide at a point approximately an integral number of half wavelengths from said series junction at said second frequency, said lter functioning to pass signals at said first Vfrequency while presenting substantially a short circuit to signals at said second frequency, and a third waveguide coupling said third cavity resonator to said buncher cavity resonator.

References Cited in the tile of this patent UNITED STATES PATENTS 2,452,566 Hansen Nov. 2, 1948 2,484,798 Bradley Oct. 11, -1949 2,517,731 Sproull Aug. 8, 1950 2,601,539 Marcurn June 24, 1952 2,616,036 Adler Oct. 28, 1952 

